Continuously variable gain radio frequency driver amplifier having linear in decibel gain control characteristics

ABSTRACT

A radio frequency (RF) driver amplifier system and method that provides linear in decibel gain control is provided. The RF driver amplifier system comprises a linear transconductor receiving an input voltage and providing a controlled current based on input voltage received, temperature compensation circuitry for varying current from the linear transconductor according to absolute temperature, an exponential current controller receiving current varied according to temperature and providing an exponential current in response, and an inductive degeneration compensator receiving exponential current and providing a control current to driver amplifier circuitry, thereby compensating for inductive degeneration due to at least one inductor in the driver amplifier circuitry. Control current passes from the inductive degeneration compensator to the driver amplifier circuitry. Output gain from the driver amplifier circuitry varies linearly in decibels with respect to the input voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Utility application Ser. No.10/700,860 entitled “Continuously Variable Gain Radio Frequency DriverAmplifier Having Linear in Decibel Gain Control Characteristics” andfiled on Nov. 3, 2003 and claims the benefit of U.S. ProvisionalApplication Ser. No. 60/426,154 filed on Nov. 13, 2002 and U.S.Provisional Application Ser. No. 60/476,311 filed on Jun. 6, 2003.

BACKGROUND

1. Field

The present invention relates generally to the field of communicationsand more specifically to providing specific gain control characteristicsin Radio Frequency (RF) driver amplifiers (DAs).

2. Background

In a wireless communications system, a user having a terminal (e.g., acellular phone) communicates with another user via transmissions on thedownlink (forward link) and the uplink (reverse link) through one ormore base stations. Downlink refers to transmission from the basestation to the terminal, while uplink refers to transmission from theterminal to the base station.

Cellular telecommunications systems, such as Code Division Multipleaccess (CDMA) communications systems, are often characterized by aplurality of mobile stations, or terminals (e.g. cellular telephones,mobile units, wireless telephones, or mobile phones) in communicationwith one or more Base Station Transceiver Subsystems (BTSs). Signalstransmitted by the mobile stations are received by a BTS and oftenrelayed to a Mobile Switching Center (MSC) having a Base StationController (BSC). Alternately, mobile station transmissions may bereceived by a BTS and relayed to a Public Data Serving Node (PDSN)through a BSC. The MSC and the PDSN, in turn, route the signal to aPublic Switched Telephone Network (PSTN), a data network, or to anotherterminal. Similarly, a signal may be transmitted from the PSTN or datanetwork to a terminal via a base station or BTS and an MSC, or via aBTS, a BSC, and a PDSN.

The output stage of a wireless communication device, or terminal,employed in connection with the foregoing wireless communication systemtypically includes amplifiers that strengthen the radio frequency (RF)transmission in the foregoing system. For example, the wirelesscommunication device may be a CDMA wireless communication device orterminal that employs one or more RF amplifiers to provide an adequateradar frequency signal transmission.

In a direct conversion transmitter architecture, controlling RF driveramplifier gain is generally desirable for a variety of reasons. Forexample, CDMA standards require a transmitter having approximately 90 dBof gain control range. Typical high volume, manufacturable single stageVariable Gain Amplifier (VGA) circuitry can only attain in the range ofapproximately 60 dB of gain control range. As the VGA is typicallylocated at the output of the direct upconverter in the design shown, itcan be difficult if not impossible to increase the gain range in thepresence of previous VGA circuitry.

Linear in decibel gain control characteristics provide certainadvantages in CDMA applications. Power control requirements in CDMA, forexample, require tight control over the output power of the terminal.Phone output power is preferably calibrated and repeatable against thereceived power control voltage. Total average power consumption ispreferably kept to a minimum, and power consumption can be reduced inthe presence of a variable gain at the RF driver amplifier.

When implementing a gain control function in a driver amplifier,adequate linearity and noise performance must be available whendelivering a significant level of output power, such as in the range ofapproximately 10 dBm. Linear in decibel gain control is particularlydifficult due to package parasitics and bondwire inductances in theintegrated circuit design of the driver amplifier.

Previous RF systems seeking enhanced gain control over the 90 dB rangehave employed different designs with mixed results. For example, certaindesigns use multiple VGAs, as a single VGA only generally provides 40 to60 dB of gain control. A dual VGA setup can increase the gain controlrange, but this design is difficult to operate at a single frequency inthe desired operational frequency range, and is difficult to tune, bias,and calibrate properly. A dual VGA system can be used for a dualconversion (superheterodyne) architecture, as each VGA can operate at adifferent frequency in such a design. Such a design can be undesirablebecause of current drain, additional required circuitry and more complexcircuitry, requires more area, and is more expensive in an IC circuitdesign. Generally speaking, any design employing multiple VGA circuitsor designs having the effect of multiple VGA circuits are undesirable indirect conversion systems in particular.

Previous designs have also employed transmit integrated circuits withinthe terminal to use a variable bias current in the drive amplifier.Variation in the output current has been observed to be on the order offour to one over the full gain control range. While this can reducecurrent drain at low output power levels, the gain is not varied in anappreciable manner in this implementation.

It would therefore be advantageous to provide a RF driver amplifierlinear in decibel variable gain design for direct conversion transmitterapplications, particularly variable linearly over the range of zero to90 dB that overcomes the drawbacks of previous designs.

SUMMARY

The aspects described herein are directed to a radio frequency (RF)driver amplifier system that provides linear in decibel gain control.According to one aspect of the present design, the linear in decibelgain control is provided in response to gain control voltage received.The RF driver amplifier system has driver amplifier circuitry includinga bipolar junction transistor and a matching circuit. The RF driveramplifier system comprises a linear transconductor receiving an inputvoltage and providing a controlled current based on input voltagereceived, temperature compensation circuitry for varying current fromthe linear transconductor according to absolute temperature, anexponential current controller receiving current varied according totemperature from the temperature compensation circuitry and providing anexponential current in response, and an inductive degenerationcompensator receiving exponential current from the exponential currentcontroller and providing a control current to the driver amplifiercircuitry compensating for inductive degeneration due to at least oneinductor in the driver amplifier circuitry. According to this aspect ofthe design, control current passes from the inductive degenerationcompensator to the driver amplifier circuitry and bipolar junctiontransistor and matching circuit. Output gain from the driver amplifiercircuitry varies linearly in decibels with respect to the input voltage.

According to a second aspect of the present design, there is provided anapparatus for providing linear in decibel gain control based on avoltage received. The apparatus comprises a voltage to current converterthat converts the voltage received into a current, a temperaturecompensation circuit that compensates the current for temperaturechanges into a temperature compensated current, and an exponentialcurrent control and inductive degeneration compensation circuit thatreceives the temperature compensated current and removes inductivedegeneration effects to provide a reference current used to providelinear in decibel gain control.

According to a third aspect of the present design, there is provided asystem for providing linear in decibel gain control for an RF driveramplifier, comprising means for providing a current, means fortemperature compensating the current into a temperature compensatedcurrent, means for exponentially controlling the temperature compensatedcurrent into an exponentially controlled current, and means forcompensating for inductive degeneration of the exponentially controlledcurrent, thereby producing a reference current used to provide linear indecibel gain control.

According to a fourth aspect of the present design, there is provided amethod for providing linear in decibel gain control in an RF driveramplifier, comprising generating a current, temperature compensating thecurrent into a temperature compensated current, and exponentiallycontrolling the temperature compensated circuit into an exponentiallycontrolled current.

According to a fifth aspect of the present design, there is provided amethod for providing variable gain RF drive amplification to driveramplifier circuitry comprising at least one inductor. The variable gainis substantially linear in decibel gain control with respect to areceived input voltage. The method according to this aspect comprisesgenerating a current control signal, comprising receiving the inputvoltage and converting the input voltage to a current, compensating thecurrent for temperature effects by varying the current according toabsolute temperature to produce a temperature compensated current,providing a controlled exponential current based on the temperaturecompensated current, and compensating for inductive degeneration in thecontrolled exponential current, the compensating comprising altering thecurrent to address high current effects for at least one inductor in thedriver amplifier circuitry. The result of the compensating is creationof a control current passed to the driver amplifier circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates a general block diagram of an output stage for awireless communication device or terminal;

FIG. 2 shows an RF driver amplifier and its associated bias circuits;

FIG. 3 is a plot of nonideal RF driver amplifier gain controlcharacteristic as contrasted with ideal linear characteristic;

FIG. 4 illustrates a gain control block diagram that receives a controlvoltage and provides a reference current to the RF driver amplifier inaccordance with the present invention;

FIG. 5 is one aspect of a circuit that may be employed as the linearGm/voltage to current converter of the present design;

FIG. 6 illustrates one aspect of a circuit that may be employed as thePTAT (proportional to absolute temperature) temperature compensationblock of the present design;

FIG. 7 shows one aspect of a circuit that may be employed as theexponential current control block of the present design; and

FIG. 8 is an aspect of a circuit that may be employed as an inductivedegeneration compensation block of the present design.

DETAILED DESCRIPTION

A typical output stage 10 for a wireless communication device isillustrated in FIG. 1 and comprises an RF driver amplifier 12, atransmit filter 14, and an RF power amplifier 16. An output port of theRF driver amplifier 12 is coupled to an input port of the transmitfilter 14. Similarly, an output port of the transmit filter 14 iscoupled to an input port of the RF power amplifier 16. An input 18 tothe RF driver amplifier 12 originates from other circuitry notillustrated in FIG. 1. Those skilled in the art will appreciate thatother circuitry may include, for example, processing circuitry such as amodulator. In a CDMA environment, additional circuitry not present inFIG. 1 may include CDMA processing circuitry and a CDMA modulatorcircuit, among other elements.

An output from the RF power amplifier 16 may be coupled to a duplexer20, the output of which is coupled to an antenna circuit (not shown),which may include an antenna (also not shown). The duplexer 20 permitsthe antenna to be used for both transmission and reception of radiofrequency signals. In one exemplary aspect of the device, the transmitfilter 14 may be a bandpass filter selected to match the frequency rangeof operation of the wireless communication device. The transmit filter14 may be implemented as a SAW filter or ceramic filter.

FIG. 2 illustrates the details of a typical RF driver amplifier, such asmay be employed as RF driver amplifier 12. The circuitry shown providesthe necessary bias and control circuitry required to enable the RFdriver amplifier 200 to operate across the entire control range requiredby the CDMA standard. Input voltage 202 is converted to a controlcurrent at control current source 203, which is connected to thecollector of reference bipolar junction transistor (BJT) 201. A line 204is provided to connect the base of reference BJT 201 to control currentsource 203. The base of reference BJT 201 and line 204 connect to unitygain operational amplifier circuit 220 including amplifier 205, NMOStransistor 219, resistors 206 and 207 and capacitor 208. The gate ofNMOS transistor 219 connects to the output of amplifier 205, the sourceties to resistor 206, and the drain connects to VDD 202. The RF inputsignal passes to capacitor 209, which combines with the signal fromunity gain operational amplifier circuit 220 and passes to singleinductively degenerated common emitter BJT 210. Reference voltage 211 isconnected to bypass capacitor 212 and P inductor 213, which joins to thecollector of single inductively degenerated common emitter BJT 210. Theemitter from single inductively degenerated common emitter BJT 210connects to emitter degeneration inductor 214 and subsequently toground. The collector of single inductively degenerated common emitterBJT 210 is connected to the RF output via a matching circuit thatincludes Bond inductor 215, S capacitor 216, and intermediate groundconnections via Pad capacitor 217 and Bd capacitor 218. The specificvalues used for the capacitance, inductance, and resistance of FIG. 2depend on the application but may be readily determined by one skilledin the art.

The circuit of FIG. 2 operates as follows. The system generates acontrol current and forces the control current into reference bipolarjunction transistor 201. The current at the collector of referencebipolar junction transistor 201 causes the circuit to generate abase-emitter voltage, Vbe, according to the following natural logequation:Vbe=V _(t)*ln(Ic/Is)  (1)where V_(t) is the thermal voltage of the reference bipolar junctiontransistor 201. V_(t) is equal to k*T/q, where k is Boltzmann'sconstant, T the temperature of the reference bipolar junction transistor201 in degrees Kelvin, and q the electron charge. In Equation (1), Is isthe saturation current, and Ic is the collector current at referencebipolar junction transistor 201, which is the forward active saturationcurrent of reference bipolar junction transistor 201. Is is a functionof the area of reference bipolar junction transistor 201 for a givenintegrated circuit fabrication process and is approximately constant fora given transistor. The base-emitter voltage Vbe of the referencebipolar junction transistor 201 may be buffered by the unity gainoperational amplifier circuit 220 formed by amplifier 205, resistors 206and 207, capacitor 208, and NMOS transistor 219. The system applies theunity gain buffered voltage to the base of the single inductivelydegenerated common emitter BJT 210.

The system controls the output device collector current by varying thebase voltage of single inductively degenerated common emitter BJT 210.This base voltage may be varied by forcing a specific control currentfrom a current source such as control current source 203 connected tothe reference bipolar junction transistor 201 as shown. The systemapplies the base voltage to the output device through a unity gainbuffer such as in the manner shown in FIG. 2.

The gain of the terminal is critical to operation. Generally, overallvoltage gain at the terminal is proportional to transconductance and isa function of the DC bias of the circuit:G=g _(m) *R _(out)  (2)where G is gain at the terminal, g_(m) is the transconductance of thecommon emitter BJT neglecting inductive feedback, and R_(out) is outputresistance. This is true for low frequencies. g_(m) varies as shown inEquation (3):g _(m) =Ic/V _(t)  (3)where Ic is again collector current and V_(t) is thermal voltage ofsingle inductively degenerated common emitter BJT 210. As collectorcurrent follows the relationship:Ic=Is*e ^((Vbe/V) ^(t) ⁾  (4)the Vbe of reference bipolar junction transistor 201 and the RFtransistor, single inductively degenerated common emitter BJT 210, aretypically identical. As a result, the collector currents of referencebipolar junction transistor 201 and single inductively degeneratedcommon emitter BJT 210 will generally be proportional to one another.Saturation current, Is, of the single inductively degenerated commonemitter BJT 210 may be X times larger than that of reference bipolarjunction transistor 201, where X is the ratio of emitter areas betweenreference bipolar junction transistor 201 and the single inductivelydegenerated common emitter BJT 210.

In general at high frequencies, the gain of amplifier 205 isGain=GmZ_(L), where Gm is the transconductance of single inductivelydegenerated common emitter BJT 210 with emitter degeneration inductanceincluded, and Z_(L) is the output impedence seen at the collector ofsingle inductively degenerated common emitter BJT 210. This outputimpedence is the parallel combination of the load impedence reflectedfrom the RF output back through the matching circuit of FIG. 2 and theoutput device itself. In this arrangement:Gm=gm/(1+gm*Ze)  (5)where gm represents the bipolar transconductance and is equal toIc/V_(t). Ze is the value of the impedence of the emitter degenerationinductor 214, which is equal to 2*π*frequency*L, where L is the value ofemitter degeneration inductor 214.

At small collector current values, gm*Ze is much less than 1. Thisrelationship causes the RF amplifier Gm to be equal to or approximatelyequal to the bipolar transconductance gm. Gm represents thetransconductance of single inductively degenerated common emitter BJT210 with emitter degeneration inductance included. This indicatesamplifier gain is approximately proportional to the collector current atsmall values of gm*Ze. For gm*Ze being much greater than 1, the value ofGm approaches 1/Ze. Thus for very large collector currents, the gain ofsingle inductively degenerated common emitter BJT 210 is approximatelyconstant and equal to Z_(L)/Ze.

If the value of the controlling current is not properly generated in thedesign of FIG. 2, the nonideal characteristic of plot 301 may result,which represents a departure from the ideal linear in dB characteristicof plot 302. Other nonideal curves may result instead of the plot 301illustrated in FIG. 3. Depending on the terminal and application, theplot 301 is therefore measured at multiple points, typically in excessof ten points across the power control range, to determine performanceand provide the ability to accurately calibrate the terminal.

In the present design, the control current is varied to provide linearin decibel performance for the RF driver amplifier 200. The systemderives this varied control current by obtaining an input voltage from avoltage source such as the MSM (Mobile Station Modem) and converting thevoltage to the control current using different compensation techniques.

The automatic gain control voltage to control current conversion isillustrated in block diagram form in FIG. 4. From FIG. 4, the automaticgain control voltage passes to linear Gm block 402. Linear Gm block 402in this aspect converts the voltage Vagc from node 401 to a differentialcurrent. The differential aspect of linear Gm block 402 is optional, andthe required aspect of linear Gm block 402 is the ability to convertvoltage to current. Various devices may be employed to convert voltageto current, including but not limited to a transconductance amplifier orother transconductor, and other devices known to those skilled in theart able to linearly convert voltage into current and potentiallydifferential current. In this aspect, differential current flows fromlinear Gm block 402 to temperature compensation block 403, whichperforms temperature compensation for the current received based onabsolute temperature of the device, in this case the differentialcurrent received from linear Gm block 402. The temperature compensateddifferential currents are provided to exponential current control block404, which uses aspects of the exponential collector current propertiesof the bipolar transistors in the gain control circuits to compensatefor the exponential aspects of the collector current in the RF driveramplifier circuitry 200. The result of the exponential current controlblock 404 passes to inductive degeneration compensation block 405, whichmodifies the current from exponential current control block 404 tocompensate for inductive degeneration and enable linear in decibel gaincontrol. The result of the inductive degeneration compensation block 405is a reference current or control current Icontrol, which is provided ascurrent at control current source 203 shown in FIG. 2.

One circuit that may be employed to convert voltage to current as inlinear Gm block 402 is illustrated in FIG. 5 and operates using voltagefrom the MSM. From FIG. 5, the Vcontrol signal is converted into acurrent using the following equation:Vcontrol/R 1=Icontrol  (6)

Vcontrol is the control voltage applied to the positive terminal ofamplifier 502, R1 the resistance of resistor 503, and Icontrol thecontrol current resulting from NMOS transistor 501. The output ofamplifier 502 connects to the gate of NMOS transistor 501. From FIG. 5,the negative terminal of amplifier 502 receives feedback from the sourceof NMOS transistor 501, and the source also feeds resistor R1, which isconnected to ground. Icontrol varies linearly with the value ofVcontrol. This control current Icontrol and control voltage Vcontrol areused in other aspects of the design as described below.

Collector current in a BJT varies according to temperature, and thepresent system compensates the current for temperature differences usingtemperature compensation block 403. Temperature compensation block 403provides a PTAT (proportional to absolute temperature) conversionfunction. The function of temperature compensation block 403 is toprovide an increase in current when temperature increases.Operationally, current is compensated by multiplying the currentreceived from linear Gm block 402 by a temperature compensation factorderived from the then-existing temperature. One circuit that may beemployed for temperature compensation in the present scheme isillustrated in FIG. 6. The circuitry in FIG. 6 provides the followingfunctionality:Ip _(control) =Icontrol*(I _(PTAT) /I _(ref1))  (7)where Icontrol is the control current generated from FIG. 5 and I_(ref1)is a constant current that does not vary with temperature or any otherparameter.

As shown in FIG. 6, voltage VDD is applied at the top of the circuit inthe configuration shown, and this voltage passes to control currentsource 601, PTAT current BJT 602, and reference 1 current BJT 603.Output from the emitter of PTAT current BJT 602 passes to PTAT currentsource 605, which is connected to ground, and to the base of controlcurrent BJT 604. Output from control current source 601 passes to thebase of PTAT current BJT 602, the base of reference 1 current BJT 603,and to the collector of control current BJT 604. The emitter from PTATcurrent BJT 602 passes to the base junction of control current BJT 604.The emitter from reference current BJT 603 passes to reference 1 currentsource 606, which is connected to ground. The emitter from reference 1current BJT 603 is also connected to the base of Ip_(control) currentBJT 607, which receives the Ip_(control) current at its collector andhas its emitter connected to ground.

According to the arrangement shown in FIG. 6, and based on Equation (7),the PTAT current at PTAT current source 604 and reference 1 current atreference 1 current source 606 provide temperature compensation valuesfor the differential current received. Icontrol is multiplied by thecurrent at PTAT current source 604 divided by the current at reference 1current source 606. From this circuit, it may be appreciated that PTATcompensation is roughly equivalent to taking (Vt/Vcontrol)*Icontrol,which provides Icontrol with a PTAT temperature characteristic.

Maximum compensated current, Imax_(comp), is computed according to thefollowing equation:Imax_(comp) =Vcontrol _(max) /R 1*(I _(PTAT) /I _(ref1))  (8)where Vcontrol_(max) is the maximum value of Vcontrol, R1 the resistanceused in FIG. 5, and I_(PTAT) and I_(ref1) as shown in FIG. 6.

From FIG. 7, the linear PTAT control currents Ipcontrol and Imax_(comp)are converted to a temperature compensated exponential control currentwith a bipolar differential pair as follows: $\begin{matrix}{I_{lindB} = {I_{ref2}{\mathbb{e}}^{\lbrack\frac{{Ipcontrol}\quad*{R2}}{V_{t}}\rbrack}}} & (9)\end{matrix}$where I_(lindB) is the linear in dB current which now variesexponentially with Ipcontrol, I_(ref2) is the reference currentillustrated in FIG. 6, Ipcontrol is the linear PTAT control current fromFIG. 6, V_(t) the thermal voltage, and R2 the resistances of resistor710 or 711 of FIG. 7. Equation (9) provides a reference for linear in dBcontrol of the DA collector current with small feedback, with anemphasis toward enabling the V_(t) in the denominator to be cancelled.Ipcontrol is proportional to V_(t) due to the previous temperaturecompensation.

To simplify the following description, a normalization factor A isemployed based on Equation (9), where:A=I _(lindB) /I _(ref2) =e ^(((Ipcontrol*R2)/Vt))  (10)Normalization factor A corresponds to an exponentially varying currentthat results in linear in dB gain for low currents where inductivedegeneration can be neglected.

FIG. 7 illustrates a circuit that provides the exponential currentcontrol that effectively matches the exponential characteristic of thecircuit shown in FIG. 2. From FIG. 7, the linear differential PTATcurrents are converted to a temperature compensated exponential controlcurrent with a bipolar differential pair. The reference current, Iref2,is as shown, and is associated with PMOS transistor 703. The systemmultiplies reference current Iref2 by the normalization factor A at PMOStransistor 701. PMOS transistor 702 includes a bypass 704, while PMOStransistor 703 includes bypass 705. The emitter from each of PMOStransistors 702 and 703 pass to the collectors of first BJT 706 andsecond BJT 707, respectively. The currents received from temperaturecompensation block 403 are Ipcontrol and I_(MAXcomp), shown at Ipcontrolcurrent source 708 and I_(MAXcomp) current source 709, respectively.Output from these current sources pass to the bases of the BJTs,specifically the output from Ipcontrol current source 708 passes to thebase of first BJT 706 and the output from I_(MAXcomp) current source 709passes to the base of second BJT 707. Output from Ipcontrol currentsource 708 passes to resistor 710 and to ground, while output fromI_(MAXcomp) source 709 passes to resistor 711 and then to ground.Emitter from both first BJT 706 and second BJT 707 pass to summationsource 712, which then passes to ground. The value of summation source712 is (A+1)*Iref2. The exemplary circuit shown in FIG. 7 provides areference for linear in dB control of the DA collector current withsmall inductive feedback. For larger collector currents, such as thosein the range of in excess of approximately 70 percent of Vcontrol(collector currents where Gm*Ze is equal to or greater than 1), feedbackfrom inductive degeneration in the emitter causes the characteristic todepart from linear, and the control current may be further processed tocompensate for this feedback using inductive degeneration compensation.

To compensate for the inductive degeneration in the output device, anadditional inductive degeneration compensation block 405 furtherprocesses the exponential control current received from the exponentialcircuit control block 404 for large collector currents in the outputdevice. The degeneration compensation block compensates for inductivedegeneration and provides a linear in dB characteristic across the fullgain control range of the driver amplifier, primarily addressing thehigh end collector current range. Without this inductive degenerationcompensation, the gain control characteristic would be linear in dB atlow output power and plateau at high output powers.

Inductive degeneration compensation in combination with exponentialcontrol corrects deviation from linear at high collector currents. Thevoltage gain of the output device, namely the RF driver amplifier 200including its associated bias circuits, for the high collector currentsituation, is given as approximately:Av=(Gm*Z _(L))/(1+Gm*Z _(E))  (11)where Gm is the transconductance of single inductively degeneratedcommon emitter BJT 210 with emitter degeneration inductance included,Z_(L) is the load impedance seen at the collector, Z_(E) is emitterinductor impedence, and Av the voltage gain. Once again, normalizationfactor A may be employed where Av is the voltage gain and Amax is themaximum gain of the inductively degenerated transconductor:A=Av/Amax  (12)withAmax=Z _(L) /Z _(E)  (13)Note that from Equation (12), normalization factor A is equal to onewhen Av is equal to Amax. From Equations (11) through (13), collectorcurrent is represented by:Ic=(V _(t) /Z _(L))*Amax(A/(1−A))  (14)The collector current that may be employed for constant normalized gainis PTAT, and thus I_(ref3) is a PTAT current. Equation (14) is the biascurrent used to generate the linear in dB characteristic.

Again, using normalization factor A where:A=I _(lindB) /I _(ref3) =e ^(((Ipcontrol*R2)/Vt)) =Av/Amax  (15)the collector current in terms of normalization factor A is:I _(DAcontrol)=(I _(ref3) *A*I _(ref2))/(I _(ref2)*(1−A))=I_(ref3)(A/(1−A))  (16)I_(ref3) is assumed to operate as PTAT to compensate for PTAT variationof gm of the BJT. Thus I_(ref3) represents a reference current thatprovides a constant gain versus temperature, process, and voltage. Theforegoing set of equations ignores certain nonideal effects, includingbut not limited to rB and finite β of a typical BJT.

The translinear circuit presented in FIG. 8 can be used to implement theinductive degeneration compensation function described in Equation (16).This circuit takes the output of FIG. 7, A*Iref2, which generates anexponentially varying control current for low currents and modifies theoutput to compensate for inductive degeneration at high currents. Thecombination of these two circuits enables linear in dB control acrossthe entire gain control range. From FIG. 8, VDD is applied at the top ofthe circuit and enters the collectors in first BJT 801, second BJT 802,fifth BJT 805, and sixth BJT 806, as well as the bases of first BJT 801and sixth BJT 806. The emitter from first BJT 801 passes to Iref3current source 811 and the base of second BJT 802. The emitter of secondBJT 802 passes to A*Iref2 current source 812 and to the base of thirdBJT 803. The emitter of sixth BJT 806 passes to the base of fifth BJT805 and the collector of third BJT 803. Fourth BJT 804 receivesIDAcontrol at its collector and passes signal from its emitter, whichcombines with the signal from the emitter of third BJT 803 into Iocurrent source 813. The emitter of fifth BJT 805 provides the current(1−A)*Iref2 which combines with the output of A*Iref2 current source 815to form the current Iref2 at Iref2 current source 814. The current(1−A)*Iref2 is also provided to the base of fourth BJT 804. A*Iref2current source 815 receives the VDD voltage. The circuit may be used asblock 405 in FIG. 4, and it provides a current limit, Io, that can avoidthe singularity due to the presence of (1−A) in the denominator ofEquation (16).

Adding gain control (30-40 dB) to the driver amplifier in the mannershown allows the Upconverter VGA, namely RF driver amplifier 200 fromFIG. 2, to employ a robust, highly manufacturable topology with only inthe range of approximately 60 dB of gain control. A linear in dB gaincontrol characteristic may be achieved in this manner to first order inthe RF driver amplifier 200 by biasing the RF driver amplifier with theexponentially varying collector current. As a result, the RF driveramplifier 200 carries a significant DC bias at rated output power inorder to meet the distortion requirements of the CDMA standard. Foroutput powers less than the maximum, the bias of the driver amplifierdrops exponentially, saving considerable current drain and reducingtotal terminal power consumption.

For minimum output power conditions, the present design biases theoutput device at a very low collector current. This biasing can place RFdriver amplifier 200 in an attenuation mode at low output power, therebyoffering the ability to suppress noise and spurious signals and providea better signal to noise ratio at low output power. This is increasinglyimportant in 3G standards. It can also be very useful in suppressing LOfeedthrough in direct upconversion transmitter architectures. The gaincontrol bias circuitry described herein operates in a continuous fashionacross the gain control range. As a result, there are minimaldiscontinuities in the gain control characteristic, enabling the closedloop power control used in a CDMA system to function properly. The gaincontrol also allows the power consumption to be optimized to the CDMAprofile. At high power the bias is large as required for linearity. Forlower powers it drops off approximately exponentially. The gain controlallows the RF driver amplifier 200 to enter attenuation mode at lowoutput power, thus suppressing noise and spurious products at the outputof the transmitter.

The foregoing discusses the linear in dB control and presents a samplecircuit for high collector currents. For low collector currents, thegain of the single inductively degenerated common emitter BJT 210 ofFIG. 2 is approximately:gm*Z _(L)=(Ic*Z _(L))/V _(t)  (17)where gm is transcondance, Ic the collector current, Z_(L) the outputimpedence seen at the collector of the single inductively degeneratedcommon emitter BJT 210, and V_(t) the thermal voltage of singleinductively degenerated common emitter BJT 210. In decibels, the gain isgiven by:Gain(dB)=10*log((Ic*Z _(L))/V _(t))  (18)Thus the gain varies logarithmically with the collector current at lowcollector current levels.

The collector current will vary exponentially with respect to a linearlyvarying Vbe asIc=Is*e ^((Ve/Vt))  (19)Substituting the Ic value of Equation (19) into the expression ofEquation (18) produces:Gain(dB)=10*log((Is*e ^((Vbe/Vt)))*Z _(L) /V _(t))  (20)Equation (20) yields the following relationship between the gain indecibels and Vbe:Gain(dB)=10*(Vbe/V _(t))+log(Is*Z _(L) /V _(t))  (21)The log term of Equation (20) is approximately constant and the gain indB varies linearly with Vbe. This relationship may be employed to offerlinear in dB control for low collector currents and modified to allowlinear in dB control at high collector currents.

The previous description of the embodiments of the invention is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A circuit for providing linear in decibel gain control in response togain control voltage, the circuit adapted to couple to a driveramplifier circuitry including a bipolar junction transistor and amatching circuit, the circuit comprising: a linear transconductorreceiving an input voltage and providing a control current based oninput voltage received; temperature compensation circuitry for varyingcurrent from the linear transconductor according to absolutetemperature; an exponential current controller receiving current variedaccording to temperature from the temperature compensation circuitry andproviding an exponential current in response; and an inductivedegeneration compensator for receiving exponential current from theexponential current controller and providing a control current to thedriver amplifier circuitry compensating for inductive degeneration dueto at least one inductor in the driver amplifier circuitry, the controlcurrent passing from the inductive degeneration compensator to thebipolar junction transistor and the matching circuit of the driveramplifier circuitry to generate an output gain that varies linearly indecibels with respect to the input voltage.
 2. The circuit of claim 1,wherein the linear transconductor converts the input voltage to adifferential current.
 3. The circuit of claim 2, wherein the temperaturecompensation circuitry compensates the differential current fortemperature effects by varying the differential current according toabsolute temperature.
 4. The circuit of claim 3, wherein the exponentialcurrent controller comprises a bipolar differential pair that convertsthe differential current into the exponential current.
 5. The circuit ofclaim 1, wherein the exponential current controller and the inductivedegeneration compensator correct deviation from linear performance forhigh collector currents.
 6. The circuit of claim 1, wherein theinductive degeneration compensator employs a translinear circuit.
 7. Thecircuit of claim 1, wherein the temperature compensation circuitryprovides PTAT compensation using bipolar junction transistor circuitry.8. The circuit of claim 7, wherein the inductive degenerationcompensator employs a translinear circuit comprising bipolar junctiontransistor circuitry.
 9. A circuit for providing linear in decibel gaincontrol based on a voltage received, the circuit adapted to couple to avoltage to current converter that converts the voltage received into acurrent, the circuit comprising: a temperature compensation circuit thatcompensates the current for temperature changes into a temperaturecompensated current; and an exponential current control and inductivedegeneration compensation circuit that receives the temperaturecompensated current and removes inductive degeneration effects toprovide a reference current used to provide linear in decibel gaincontrol.
 10. The circuit of claim 9, wherein the voltage to currentconverter is of the type that converts the voltage received into adifferential current.
 11. The circuit of claim 10, wherein thetemperature compensation circuit compensates the differential currentfor temperature effects by varying the differential current according toabsolute temperature.
 12. The circuit of claim 11, wherein theexponential current control and inductive degeneration compensationcircuit comprises a bipolar differential pair that converts thedifferential current into the reference current.
 13. The circuit ofclaim 9, wherein the exponential current control and inductivedegeneration compensation circuit corrects deviation from linearperformance for high collector currents.
 14. The circuit of claim 9,wherein the exponential current control and inductive degenerationcompensation circuit employs a translinear circuit.
 15. The circuit ofclaim 9, wherein the temperature compensation circuit provides PTATcompensation using bipolar junction transistor circuitry.
 16. Thecircuit of claim 15, wherein the exponential current control andinductive degeneration compensation circuit employs a translinearcircuit comprising bipolar junction transistor circuitry.
 17. Anintegrated circuit (IC) for providing linear in decibel gain control foran RF driver amplifier, comprising: means for temperature compensating acurrent into a temperature compensated current; means for exponentiallycontrolling the temperature compensated current into an exponentiallycontrolled current; and means for compensating for inductivedegeneration of the exponentially controlled current to generate areference current to provide linear in decibel gain control.
 18. The ICof claim 17, wherein the current is a differential current.
 19. The ICof claim 18, wherein the temperature compensating means compensates thedifferential current for temperature effects by varying the differentialcurrent according to absolute temperature.
 20. The IC of claim 19,wherein the exponentially controlling means comprises a bipolardifferential pair that converts the differential current into theexponentially controlled current.
 21. The IC of claim 17, wherein theexponentially controlling means and the inductive degenerationcompensating means correct deviation from linear performance for highcollector currents.
 22. The IC of claim 17, wherein the inductivedegeneration compensating means comprises a translinear circuit.
 23. TheIC of claim 17, wherein the temperature compensating means provides PTATcompensation using bipolar junction transistor circuitry.
 24. The IC ofclaim 23, wherein the inductive degeneration compensating means employsa translinear circuit comprising bipolar junction transistor circuitry.25. A method of providing linear in decibel gain control in an RF driveramplifier, comprising: receiving a current; temperature compensating thecurrent into a temperature compensated current; exponentiallycontrolling the temperature compensated circuit into an exponentiallycontrolled current; and compensating for inductive degeneration of theexponentially controlled current to generate a reference current toprovide linear in decibel gain control.
 26. The method of claim 25,further comprising applying the reference current to an RF driveramplifier circuit.
 27. The method of claim 26, wherein the current is adifferential current.
 28. The method of claim 27, wherein thetemperature compensating compensates the differential current fortemperature effects by varying the differential current according toabsolute temperature.
 29. The method of claim 28, wherein theexponentially controlling comprises employing a bipolar differentialpair for converting the differential current into the exponentiallycontrolled current.
 30. The method of claim 26, wherein theexponentially controlling and the inductive degeneration compensatingcorrect deviation from linear performance for high collector currents.31. The method of claim 26, wherein the inductive degenerationcompensating comprises employing a translinear circuit.
 32. The methodof claim 26, wherein the temperature compensating provides PTATcompensation using bipolar junction transistor circuitry.
 33. The methodof claim 32, wherein the inductive degeneration compensating employs atranslinear circuit comprising bipolar junction transistor circuitry.34. A method for providing variable gain RF drive amplification todriver amplifier circuitry including at least one inductor, the variablegain being substantially linear in decibel gain control with respect toan input voltage from which is generated a current, comprising:compensating the current for temperature effects by varying the currentaccording to absolute temperature to produce a temperature compensatedcurrent; providing a controlled exponential current based on thetemperature compensated current; and compensating for inductivedegeneration in the controlled exponential current, the compensatingcomprising altering the current to address high current effects for atleast one inductor in the driver amplifier circuitry and generating acontrol current passed to the driver amplifier circuitry.
 35. The methodof claim 34, wherein the current is a differential current derived fromthe voltage, and wherein the compensating comprises compensating thedifferential current for temperature effects by varying the differentialcurrent according to absolute temperature to produce the temperaturecompensated current.
 36. The method of claim 35, wherein the providingthe controlled exponential current comprises converting the differentialcurrent into the temperature compensated current using a bipolardifferential pair.
 37. The method of claim 34, wherein the providing ofcontrolled exponential current and inductive generation compensatingcorrects deviation from linear performance for high collector currents.38. The method of claim 34, wherein the providing of controlledexponential current and inductive generation compensating employs atranslinear circuit.
 39. The method of claim 34, wherein compensatingthe current for temperature effects comprises providing PTATcompensation using bipolar junction transistor circuitry.
 40. The methodof claim 39, wherein compensating for inductive degeneration in thecontrolled exponential current comprises employing a translinear circuithaving bipolar junction transistor circuitry.
 41. A circuit adapted tocouple to a linear transconductor from which is generated a controlledcurrent based on input voltage received, the IC comprising: temperaturecompensation circuitry for varying current from the lineartransconductor according to absolute temperature; an exponential currentcontroller receiving current varied according to temperature from thetemperature compensation circuitry and providing an exponential currentin response; and an inductive degeneration compensator receivingexponential current from the exponential current controller andgenerating a control current to compensate for inductive degeneration.42. The circuit of claim 41, wherein the controlled current is adifferential current.
 43. The circuit of claim 42, wherein thetemperature compensation circuitry compensates the differential currentfor temperature effects by varying the differential current according toabsolute temperature.
 44. The circuit of claim 43, wherein theexponential current controller comprises a bipolar differential pairthat converts the differential current into the exponential current. 45.The circuit of claim 41, wherein the exponential current controller andthe inductive degeneration compensator correct deviation from linearperformance for high collector currents.
 46. The circuit of claim 41,wherein the inductive degeneration compensator employs a translinearcircuit.
 47. The circuit of claim 41, wherein the temperaturecompensation circuitry provides PTAT compensation using bipolar junctiontransistor circuitry.
 48. The circuit of claim 41, wherein the circuitis an RF chip.